1. Field of the Invention
This invention generally relates to digital communication systems, and more specifically, to an enhanced system and method for performing feedback equalization and complementary code key (CCK) decoding using a trellis structure.
2. Prior Art
In many digital communication scenarios (e.g. telephone transmission, broadcast TV transmission, cable etc.). The transmitted signal arrives at the receiver through more than one path in addition to the direct path. This condition is called “multipath” and leads to intersymbol interference (“ISI”) in the digital symbol stream. This ISI is compensated for in the receiver through an equalizer which in many cases is a DFE as shown in FIG. 1. U.S. Pat. No. 5,572,262 shows one method of combating these multipaths.
A DFE 10 (FIG. 1) has two filter sections, a forward filter 12 and a feedback filter 16. The input to the forward filter 12 is the received data which includes the transmitted symbol sequence ak, noise nk and multipath hi. The input to the feedback filter is the quantized equalizer output âk. The output of both the sections are summed 18 to form the final equalizer output ãk 19 which is also the input to the next stage in a trellis-coded system, the trellis decoder. While a DFE performs better than a linear equalizer in severe ISI, the performance is limited by error propagation through the feedback filter 16 of the DFE 10. Error propagation occurs in the feedback filter 16 when the quantized equalizer output âk is not the same as the transmitted symbol ak. If an error is made in determining the symbol âk at the output of the slicer 14, this incorrect symbol is fed back to the input of the feedback filter 16 and propagates. As known, the slicer 14 quantizes the filtered signal, providing an estimate of the symbol received. In many systems which employ error correction codes like trellis codes and/or Reed-Solomon codes to obtain very low error rates at moderate SNRs, the “raw” symbol error rate (SER) at the equalizer output can be extremely high. For example, in a Vestigal Sideband (VSB) system, at threshold in white noise the SER at the equalizer output is about 0.2. The increased error propagation due to these high SERs can cause the DFE to lose a couple of db in performance as compared to the case of no error propagation. Additionally, the error propagation causes the error sequence at the equalizer output to be correlated, since it depends on past incorrect symbol decisions. This correlation has an adverse effect on the subsequent trellis decoder which is usually designed for a white noise sequence.
According to the IEEE 802.11b high-rate wireless communications standard implementing direct sequence spread spectra techniques, bit stream data may be encoded using a standard known as Complementary Code Keying (CCK). This CCK encoding scheme is used to achieve 5.5 Mbps or 11 Mbps in wireless LANs. Rather than using the Barker code, which is the standard 11-bit chipping sequence used to encode data bits, CCK requires that data be encoded using a series of codes called Complementary Sequences. Because there are 256 unique code words that can be used to encode the signal, up to 8 bits can be represented by any one particular code word (assuming an 11 Mbps bit stream).
It is the case that, for most symbol modulation schemes, the Decision Feedback Equalizer adequately performs notwithstanding the difficulty of providing symbol estimates âk. This is because the estimation is done on a symbol-by-symbol basis.
It would be highly desirable to provide a DFE that exploits the fact that when a symbol is received, there is a relationship between the other symbols before and after it, e.g., if it is in the middle of a CCK code word.
Past efforts have relied upon providing equalization first and, then perform the CCK decoding. However, it would be highly desirable to provide both CCK modulation decoding and equalization at the same time.